Crystallization



April 21, 1959 driginal Filed March 4, .1954

w. c; SAEMAN CRYSTALLIZATION 2" Sheets-Sheet 1 INVENTOR. WALTER C.SAEMAN April 21, 1959 w. c. 'SAEMAN 2,883,273 r v CRYSTALLIZATION v 1Original Filed Ma ch 4, i954 sheds-Sheet 2 FIG-"2' v IINVENTOR. WALTERc. SAEMAN BY A zsrm United States Patent 3 CRYSTALLIZATION waltertCmSaeman, Orange, .Conu las'signortto .Dlin sMathieson "ChemicalCorporation, .a ,.corporation ,of Mirginia "original 'application March4, 1954, Serial Nos 414,192,

noWPatent No.2,827-,366, dated March 18,1958. Di- .v ided and thisapplication February 14, 1957, 'Serial "No-"643,907

.lihistinvention relates. to ancrystallization. process and lapparatus,for producing; large. crystalline material from cturbulent ,suspensions.under controlled conditions of operation. ,More particularly.thisinven'tion relatesto crystallization process and apparatus inwhicha control exerted. overrthe'existence time and number of fineacrystalstin v,the.turbulent, suspension of the process j in isuchmanner that an zunexpectedly'high yield oflarge tptoductscrystalsgis Kproduced. Furthermore, the control .Hisaso. formulated as torenable theeconomical reuse of vonattercontained]inftheuridesired1fine crystalswith "a iiminimumguiib'alance of the optimum crystallizing con-;iiditions that exist.

j Hereto'fore, standard commercial procedures of 'the tturbulent typeifor obtaining product crystals of jlarge g size involved only a meansfor growing "and selectively removing crystals after theyhave-reached-acertain size. "-They in effect were only procedures in which a controlwastexerted over the selective -removal, --no importance being given:tothe eflect of the number of 'fine crystals "existing in "the'turbulent -suspension uponnwhich growth of material will occur. In sofailing to exert acontrol g over'i'the "number of-fine crystalseiiisting in the suspenrsion, an economical and 'optimum -production ofcoarse -crystalline-material could not result.

""r'rly one*disclosure found which at-temptstowefiect 40 Lfsuchj acontrol of a turbulent types crystallization process 'TWQSUTS.lg-845,142. *But the-process and apparatus xdisclosed in "this" patenthas 1 been found to' be impractical. for commercialoperation. Forexample,:its.useuof a z screen to separate out unwanted seed crystals.isattended with frequent clogging and "crust-mg. Also its ruse gof onaaux iliary stream for performing three l independent itunctionsi it/impractical: because it. combinesrthreestteams which require :lthreetrdifierent Fflow lv'elocities; :;namely, (1 )1 the: stream :whichwithdraws excess seed crystals from tithe QSHSPEIISiOI! requirestalihigh :diow rate I so that there his promptremoval :of ,the excess,seed crystals in as. small a size as possible, preferablytwhenithey;canwsti-llzbel con- ;sidered muclei ;:crysta1s,t(j2) sthetfiow rate.through the :dissolving .tank must besrelat-ively low to permitcrystalr-lizedxmaterial in the; tankto be. dissolved with. a minimum:1amount.ofcoverheating or overdilution so thattupon return.

[the solution.,to- ..the. crystal growth circuit, there lwill est-,nounbalanceofg the optimum .crystallizing conditions .-existi.n g therein,sand (3) the ,fiow .rate through ,the celutriationcolumn for theselective. removal of ,product .crystals a must also 1 be independentlycontrolled .so that -;-an1.eflicient regulation of .the. removal of the3 product crystal soflcertain sizeswis effected. It. has been .foundthat thisgflowi rate. should be "intermediate the rates dis- ,cussedrabove.

An; object ,of. this, invention, therefore, is a :ycrystallizaotionprocesstandflapparatus .which efiiciently exerts a control over theexistence. of .finehcrystals present in. .the turbulent suspension so asu to prevent growth upon an tunduly large anumber. ofhsuchcrystals.Another object Qof .thisainvention. is. a crystallization ,process andapparatus which economically and continuously utilizes the timaterialcontained-in suchl undesired :-1ine crystals for .iifurthercrystal,growthupon other fdesired finecrystals. ,A gstill'further objectof this. invention is a crystallization .process and apparatus;ofqtheturbulent,susp ns omiyp :which will .operate. continuouslyandwhich will produce -1. I1expectedlyllarge crystals. Other .objects willbecome apparent; to those skilled in the .art upon reading \theIfollowing disclosure.

qfienerally; speaking. in: accordance with: this invention, theforegoing objects as well as otherstwhichwill become :apparent fromtherfollowing description are. accomplished by providing an apparatus.and method fornsubstantial ly .;immediately removing undesired finecrystals ."from the turbulent suspension. circulatingin thecrystallizinggbath.

'-T,his.is accomplished byt, elu;triating the undesiredlfine crystalsfrom. the suspension. To. .QQnserve ,the material ccontained inttheseeXIractedundesiredfine crystals,gthe scrystals may :be redissolved :in.a separate quiescent ,chamberreither .byth-eat, byndding additionalsolvent,

..or by .other solvating means, Q and the resulting solution returnedto.t he .main bodyofthe liquid preferably at a tflow irate, temperatureand concentration substantially equalling .the conditions existing [inthe main body,

thereby avoiding .any .abrupt changesj in the overall Theqaccompanyingdrawings which show a certain method of carrying out the invention arenot toibe conasidered restrictive and are [shown for illustrativepurposes onlyjin which:

Figure 1..is one embodiment of anevapOratiomtype .crystallizer used in"this linvention having an outside .circulat rysvst m,

'Figure'Zis another embodiment of an evaporation-type crystallizerusedin this invention l aving an"inside circulatory system.

jInFigures 1 and 2,the main crystallization process occurs incrystallizer tankl. A body of saturatedor supersaturated liquid 2,containing suspended crystalline material contained withimthiscrystallizer tank, is constantly being circulated as more fullydescribed hereinafter andislibjected either toconcentration by anevaporationprocess using "diminished pressure and heat Chins trated in'the diagrams) or else to cooling so that a depositionofcrystallinematerial results.

Where the crystallization is effected by an evaporation process(illustrated in the drawings) "the diminished pressure 1 is prefera-blymaintained 1 partly by a vacuum line ii and partly by the use-ofasteamejectorl,which 8. The suspension is preferably kepttheatedbyeusing gonly a hot feed (9 intFigure 51,;and21lbintlfiigure Q) ,301'uelsel byi using heating elementsinithetvarious inlet and outlet pipesehereinafiter l enumerated. t Other v diminished pressure-maintainingand heat-maintaining .means mobvitouslyimayralso beuse'd. tThe vapor.6,"thatistsweptout rbyi the steammejectonis *CDIldCnSGdbYrthB :usero'fcold ccondenserispray water on similar spray condensing mediafintroduced into ..the sbarornetric condenser. at inlet 11 2andtrexpelled through outlet 12. Alternatively a, surface qcond nsercanrualso be; used in 1 .which u case. the condensed papers areutaken,ofi sin a .;separate soutlet. @Such coni, dente wap r @maytbe reused ain i des e ei he a .a solvent tennis-solving unde ired se c ystal e v.f"dissolving rah/crystalline. matter.

..Where the cry all ation. is. efie ted. byta cool n Pr e ess thecrystallizer tank need not be equipped with a pressure diminishingmeans. herefore; the crystallizer [tankfshown in either'Figurel or2,mayrbe closedratits uppermost region. The cooling is effectedpreferably by the use of a cold inlet solution coupled with coolingmeans contained in the various inlet and outlet pipes hereinafterenumerated.

In the embodiment shown in Figure l, the suspension 2 is circulated fromthe upper chamber of the crystallizer tank down through a conical falsebottom 13, attached at an intermediate point of the inner walls of thetank, and into the lower chamber which has a funnelshaped bottom. Acrystal suspension throttling valve 14, varies the size of thefunnel-shaped opening 15, for the purpose of regulating the rate of flowthrough side outlet 16 by its relationship to the main circulating flow.Sight glasses 17, permit observation of this flow through the bottomoutlet thereby facilitating adjustment of the throttling valve. With thevalve wide open the flow to the circulating pump will pass essentiallyall through the bottom outlet in view of the greater density ofsuspension over that of solution. As the valve is closed a greaterportion of the circulating stream will emerge through the side outlet16, and reach the pump via settling tank 18. In practice it is generallyfound that the valve should be maintained intermediate its full openposition and its closed position, its exact position depending upon thesize product crystal desired. In this manner, undesired fine crystalsare continuously removed from the turbulent suspension as thecrystallization process proceeds. They are redissolved in the settlingtank and the solution recirculated to the main circulatory body at arate insulticient to upset the optimum crystallizing conditions existingin the main circulatory system.

The downward flowing suspension is circulated through conduit 19,through pump, 20, and then upward through conduit 21, back into thecrystallizer tank 1. The pump is operated by motor 22. Other similarflow inducing devices such as a motor driven propeller blade obviouslymay also be used.

Into the circulatory fluid system, feed solution is introduced at inlet9, either to make up for the evaporation losses through steam ejector 4when an evaporation process is being used or else to make up the lossesdue to the intermittent opening of product crystal outlet 23. The feedsolution is introduced at such rate and under such temperatureconditions that the suspension will be induced to remain in a conditionsubstantially matching the crystallizing conditions existing in thetank. Product crystals are removed from the crystallizer through productoutlet 23 containing valve 24 for intermittent operation either manuallyor automatically.

The space beneath the conical false-bottom 11, comliquor is therebyreturned to the main suspension by conduit 27 to again take part in thecrystallizing process thereby avoiding loss of solid matter to finecrystal formation. Several valves 28 (only one shown) contained inconduits 27 (also only one shown), are used to place these tanks on andoff stream as the need arises. The undesired fine crystals obviously mayalso be dissolved in the mother liquor by other means such as by theaddition of solvent to the settling tank while it is ofi stream. Thisoff stream-on stream technique avoids the undue upsetting of thecrystallizing condition that would occur if only one settling tank wereheld on stream at all times.

In the embodiment shown in Figure 2, the suspension 2, is circulatedupwardly through tubular conduit 31, which is located axially within thecrystallizer tank 1, and held in place by several supporting members 32(only two shown) and by a vertically adjustable bracket 33. Dependingupon the needs of the process, this tubular conduit can be raised orlowered therebyregulating opening 35 by means of screwcrank 34, locatedoutside of the crystallizer tank at the upper end of adjustable bracket33. When the upward flowing suspension reaches the top it flows over theedges of the tubular conduit and then downward through a conicalfalse-bottom 36, located intermediate the crystallizer tank into thelowermost funnel-shaped chamber of the tank. After passing throughcontrollable opening 35, it again flows upwardly through axially locatedconduit 31, thereby causing an inner" circulatory system. To efiect thecirculation, an upward directing propeller 37, axially located inconduit 31 is driven by propeller shaft 38,, and motor assemblymunicates with a side outlet 16, so that the undesired fine crystals maybe removed by an elutriation method,

the size being determined by the rate of upward flow of the elutriatingzone 25, that exists immediately underneath the conical false-bottom 13,and the walls of the crystallizer tank 1. By the term elutriation ismeant the separation of fine particles from coarse particles by passingan upward flow of fluid through said mixture at a rate that induces thesmaller size particles to be in a more upward position of the flow thanthe coarse particles. Separating said zones will cause a separation ofthe fine particles from the coarse particles. The undesired finecrystals taken ofi by this elutriation are deposited in one or more finecrystal settling tanks 18 (only one of which is shown) wherein thecrystals settle to the bottom and the mother liquor is then returned tothe main circulatory system. More than one settling tank is desired sothat while one settling tank is on stream" for the purpose of settlingout undesired fine crystals the other tank or tanks containing finecrystals and mother liquor is preferably being heated to a slight extentby steam coils 26, so that small crystals will be redissolved in themother liquor. When redissolved the tank is again placed on stream,"manually or automatically, and the 39. Other means for effecting thecirculatory system obviously may also be used without departing from thespirit and scope of this invention.

Undesired fine crystals are removed from this inner circulatory systemby elutriating the undesired fine crystals upwardly between the conicalfalse bottom 36, and the walls of the crystallization tank 1, and theninto either of the fine crystal outlet conduits 40, depending upon whichis on stream as more fully described below. With conduit 31 raised toallow the free flow of suspension through opening 35, the flow throughfine crystal outlet 40, will be small and only very fine crystals willbe carried into the settling tank 41. As' conduit 31 is lowered, theproportion of circulating suspension through the fine crystal settlingtank 41, increases thereby carry: ing more fine crystals of increasingsize out of the crystal suspension. In practice, it is generally foundthat the tubular conduit should be maintained intermediate its lowermostposition and its uppermost position, its exact position depending uponthe size of product crystal desired. In this manner, undesired finecrystals are continuously removed from the turbulent suspension'as thecrystallization process proceeds.

The solution containing the unwanted fine crystals enters the finecrystal setting tank 41, wherein the crystals settle to the bottom whilethe mother liquor leaves the tank to return to the main circulating bodythrough conduit 42. When the fine crystal settling tank containssufiicient fine crystalline matter, it is placed off stream by closingvalve 43 either manually or automatically. The fine crystals are thenredissolved preferably by heating the contents of the tank with the useof heating coils 44, contained in the base of the tank. Other means ofdissolving fine crystals, such as by adding additional solvent may beemployed also. When the undesired fine crystals are redissolved, thetank is again placed on stream by manual or automatic operation sothatthe saturated solution containing dissolved crystalline matter might bere-used most economically.

Produce crystalline matter is removed from the crystallizer tank bymeans of product outlet' 45, which is opened intermittently with the useof valve 46, depending upon the size of crystallineimatter desired.Thiswopening of valve may be automatically ,performedto effect a totallycontinuous process. Use of sight,glasses 47, contained in thefunnel-shapedportionof the crystallizer tank enables the determinationof this desired. size.

.The apparatus described above is used in accordance with the followingprocedure. A heated solution or suspension of material is introducedinto the crystallizer tank at the start of the process and whenever theliquid level is deplenished beyond operability by evaporative losses orby product crystal. removed by .means of. feed inlets 9 and 10. .If theheat introduced with the solution is insufficient for evaporating. thesolvent. from the solution, auxiliary sources .of'heat connected iwiththe apparatus are used.

It is to be again noted here .that the apparatus described above maybemodified for a crystallization process by cooling. In suchevent, the.solution is preferablyintroduced in a cooled state and subsequentlyfurther cooledby cooling means located in the inlet and .outlet conduitsof the apparatus or else by, acoolingjacket.

"The solution is caused to circulate through the"outside or. the insidetype ,of circulatory system depending. upon the apparatus used, thevelocity being regulated for the most efficient crystallizingconditions,by means of the appropriate regulating devices shown'in thedrawings. ;Such velocity. regulationisinfiuenced by the followingfactors; namely, strong agitation aggravates the formation of excessnucleiwhile weak agita- Ltion allows .an excessive .accumulation ofcrystals to LSBttlCOllt on the.bottom.therebywobstructing Ithecircunlation of. suspension.

As the suspension circulates, crystals are either deposited out ofsolution, or are in the growing stage in which more crystalline matterdeposits upon the surface of existing crystals. For the purpose ofdefinition the following terms are used in referring to the variouscrystals existing in the suspension; namely crystal nuclei are thosebodies which have just been deposited out of solution, seed or finecrystal are either crystal nuclei or crystal nuclei having additionalmatter deposited upon their surface by reason of their relatively shortexistence in the suspension, and product crystals are those crystalswhich have grown to the size desired as the end product. In stages ofgrowth the above crystal definitions fall into the following order:First, there comes into being a crystal nucleus. It then grows until itis a fine crystal. If this crystal is allowed to remain in thesuspension and grow still further it is termed a seed crystal. This seedcrystal eventually becomes a product crystal, the end result of theprocess.

During the crystal deposition undesired fine crystals are constantlyremoved by an elutriating stream located below the conical false-bottomcontained in the crystallizer tank. These crystals are transported to afine crystal settling tank where they settle out while the mother liquoris returned to the main circulatory system. The tank, when sufficientlyfilled with deposited fine crystals is preferably placed oflf stream andthen the settled crystals are redissolved, either by the use of heat, bythe use of additional solvent, or other similar solvating means. Theresulting solution is then returned to the main circulating stream sothat there is an efficient use of all crystalline matter.

As the crystallization process proceeds, the product crystals that areproduced, are removed intermittently either manually or automatically byopening the product outlet. The product crystals are filtered off andthe remaining suspension may either be discarded or returned to thecrystallizer tank. An explanation for the success of this inventionfollows hereinafter, but it should be noted that this discussion is onlya discussion of what is believed to be correct. Other more plausibleexplanations may be later discovered and therefore this inventionshouldnot be, limited bythe herein offeredexplanation.

. According to angarti'cle. in"chemicalllEngineerlstHand- 'book,thirdedition, pagesl058 and 1059 (1950),,Mc-

Graw-Hill Book Co., New York,;:N..Y-,., lthesize .of, product crystalsfrom .a continuous crystallization appears ,to lbe dependent uponatleast thefollowing'four principles:

.(I) "The rate of .crystalgrowthunder turbulent .conditions. is directlyproportional to the..;supersaturationtof the solution.

(2) The: rate of .crystal growth is proportional to lithe area ofcrystal surface exposed to the solution.

(3) Eachfaceof a crystal is characterized by a rate of growthcoetficient which .determines .the geometric shape of the crystal.Cor'ollarics of this .are that ithe geometric shape of a crystalessentially ;.remains ;;un-

changed asthe crystal growsqandihatfthe .intei'facial' .;.angles of .a.crystalremain .constant.

W-I1L f"W=anL where:

.rW: cumulative .weight mar-constant From this relationship one can seethat the cumulative weight of crystals up to a given size varies as thefourth power of the crystal size. Using this fourth power relationship,the cumulative weight of relatively small seed crystals in a turbulentsuspension is found to be generally negligible as compared to the weightof the product crystals. For example, crystals smaller than half sizeconstitute only & or A of the total weight of crystals in suspension. Itcan, therefore, be seen that heretofore used commercial processes weretoo concerned with selective separation rather than other factors. Theminor proportion of undersized crystals is actually such a minorproportion of the total that they can generally be disregarded. Items ofmore importance however would appear to reside in the n, the seed rateof the relationship. The seed rate it must be controlled to realize thedesired crystal size L in the weight of product W; for example, if n isvery large for a given weight of product the size L will becomparatively small, whereas a low seed rate It will yield comparativelylarger crystals for the same weight of product. Therefore, complete sizecontrol in a crystallizer cannot be effected without controlling theseed rate n and if no means are provided for doing this the averagecrystal size L will correspond to the natural seed rate of the system. a

In accordance with this invention it has been found that in turbulentsuspension type crystallizations removing undesired fine crystalssubstantially as fast as they are formed eifects the seed rate, 12, insuch manner that the size L is proportionally greater than heretoforeobtained. The only patent that discloses such a technique is US.1,845,742 but in view of the discussion given previously of the mannerof operation used, it can be seen that commercial exploitation of thispatent was impossible. The process herein disclosed is, however, readilyadaptable to commercial operation in view of its novel mode of operationand its novel apparatus.

Among the crystals which may be readily produced by the herein disclosedprocess and apparatus the following are typical: Ammonium nitrate,ammonium sulfate, sodium sulfate, sodium chloride, potassium sulfate,and other similar-type crystalline-producing compounds. However,

the invention is obviously not to be limited by a recitation ofcrystalline-producing material for its breadth rests upon the method andapparatus rather than the specific crystal being produced.

It is to be noted that in the operation of this invention the controlover the seed rate may be varied. Should extremely large crystals bedesired the seed rate should be kept at a low rate by increasing therate of destruction of fine crystals. The throttling valve of Figure 1or the suspended conduit of Figure 2 in this case should be kept so thatits opening is small. If small product crystals are desired the finecrystal destroying rate should be kept at a minimum. The opening of thecrystal growth inducing circuit (#15 of Figure 1 and 35 of Figure 2)should be large.

While a detailed description of the invention has been provided, it isrealized that those skilled in the art may make modifications in andadaptations of the process and apparatus described above withoutdeparting from the spirit and scope of. this invention. It is,therefore, to be specifically understood that such obvious modificationsare considered within the scope of the herein described process andapparatus.

This application is a division of application Serial No. 414,192 filedMarch 4, 1954, now Patent No. 2,827,366.

The invention having thus been described, what is claimed and desired tobe secured by Letters Patent is:

1. In a crystallization process occurring in a single, vertical reactionvessel wherein there is a suspension of product size and fine crystalsnormally mixed-together in a turbulent solution, a. method ofeliminating exexcessive fine crystals from said' mixed suspensioncomprising the steps of establishing an internal quiescent zone withinsaid suspension, said zone falling within the intermediate region of thevessel, directing said fine crystals out of the mixed suspension towardssaid quiescent zone, withdrawing said fine crystals together withsolution from the zone, eliminating the withdrawn fine crystals from thesolution and returning the solution, substantially free of finecrystals, to the main body of the suspension.

2. In a crystallization process occurring in a single vertical reactionvessel wherein there is a suspension of product size and fine crystalsnormally mixed together in a turbulent solution, a method of eliminatingexcessive fine crystals from said mixed suspension comprising the stepsof establishing an internal quiescent zone within said suspension, saidzone falling within the intermediate region of the vessel, establishinga distinct current within said suspension, directing said fine crystalsout of the mixed suspension towards said quiescent zone, utilizing saidcurrent as a conveyor, withdrawing said fine crystals together withsolution from the zone, eliminating the withdrawn fine crystals from thesolution, returning clear solution substantially free of fine crystalsto the main body of the suspension and dissolving said fines andreturning the resulting solution to the main body of the suspension.

References Cited in the file of this patent UNITED STATES PATENTS1,478,337 Isaachsen Dec. 18, 1923 2,631,926 Eckstrom Mar. 17, 19532,737,451 Saeman Mar. 6, 1956

1. IN A CRYSTALLIZATION PROCESS OCCURING IN A SINGLE, VERTICAL REACTIONVESSEL WHEREIN THERE IS A SUSPENSION OF PRODUCT SIZE AND FINE CRYSTALSNORMALLY MIXED TOGETHER IN A TURBULENT SOLUTION, A METHOD OF ELIMINATINGEXEXCESSIVE FINE CRYSTALS FROM SAID MIXED SUSPENSION COMPRISING THESTEPS OF ESTABLISHING AN INTERNAL QUIESCENT ZONE WITHIN SAID SUSPENSION,SAID ZONE FALLING WITHIN THE INTERMEDIATE REGION OF THE VESSEL,DIRECTING SAID FINE CRYSTALS OUT OF THE MIXED SUSPENSION TOWARDS SAIDQUIESCENT